3GPP Long Term Evolution (LTE) is the fourth-generation mobile communication technologies standard developed within the 3rd Generation Partnership Project (3GPP) to improve the Universal Mobile Telecommunication System (UMTS) standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a User Equipment (UE) is wirelessly connected to a Radio Base Station (RBS) commonly referred to as a NodeB (NB) in UMTS, and as an evolved NodeB (eNodeB or eNB) in LTE. An RBS is a general term for a radio network node capable of transmitting radio signals to a UE and receiving signals transmitted by a UE.
FIG. 1 illustrates a radio access network in an LTE system. An eNB 101a serves a UE 103 located within the RBS's geographical area of service or the cell 105a. The eNB 101a is directly connected to the core network. The eNB 101a is also connected via an X2 interface to a neighboring eNB 101b serving another cell 105b. Although the eNBs of this example network serves one cell each, an eNB may serve more than one cell. An advantage of having one eNB serving multiple cells, is that the eNB hardware and software resources may be shared among the served cells.
In many cellular systems the physical radio resources on the air interface are shared among a plurality of active users based on their immediate need for communications. One such system is the LTE system, which will be used in the further description as an example. E-UTRAN uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink (DL) from an eNB to UEs, and discrete Fourier transform (DFT)-spread OFDM in the uplink (UL) from a UE to an eNB.
The basic LTE downlink physical radio resource may be seen as a time-frequency grid as illustrated in FIG. 2, where each resource element, i.e. each square in the grid, represents one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length 1 ms, as illustrated in FIG. 3a. One subframe is also called the Transmission Time Interval (TTI), and comprises two time slots of 0.5 ms each. The scheduling process, i.e. the process of assigning resources on the physical radio resource to the active users in a cell based on their respective need for communication, generally assigns resources for a period of one subframe, which thus constitutes a TTI. The scheduling process is therefore repeated for each TTI.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, also called Physical Resource Blocks (PRB), where a resource block corresponds to one timeslot of 0.5 ms in the time domain and twelve contiguous subcarriers in the frequency domain, as illustrated in FIG. 3b. 
One measure for handling an increasing traffic load in a cellular system is to introduce more cells. However, the difficulty in finding sites may be limiting the deployment of additional cells. One way of increasing the number of cells with a fixed set of RBS sites is the introduction of distributed RBS architecture, with distributed Radio Equipment (RE) that share the same Radio Equipment Control (REC) on the RBS site. In this approach antennas and REs 112a-f are spread to support radio communication with UEs 150a-f in different cells A-E, as illustrated in FIG. 4a, while the control of the communication with the UEs is made by the REC 114. The architecture of this type of RBS is illustrated in FIG. 4b with the REC 114 connected to the spaced apart REs 112 via a standardized interface named the Common Public Radio Interface (CPRI). The RE may provide radio communication with UEs in one or more cells. The REC communicates baseband signals and control information over the CPRI, and handles all baseband processing of transmitted and received signals, and further controls the communications in the cells. The REC may have less capacity enabled than needed to support the complete set of REs simultaneously. The CPRI allows for a flexible construction and building of a RBS.
There is thus a trend in the industry to build RBSs with the above described architecture to serve large areas. One of the main reasoning behind the trend is that it is possible to pool baseband processing resources in the REC as the traffic load often is unevenly distributed over the cells. Many cells may have no or a minor load during long durations, while other cells serving UEs that are generating high traffic are heavily loaded. The idea is thus to utilize the baseband processing resources in the REC for the cells in which the traffic is currently happening. The migration of traffic is very often on hourly basis, e.g. following the work hours. A key performance parameter in this kind of RBS is the number of cells possible to serve. A key issue is therefore to minimize the cost for a low load cell.
However, although the baseband processing resources may be more efficiently used in an RBS architecture with an REC and distributed REs, such an RBS has some disadvantages. The need for capacity on the CPRI link between the REC and the REs increases with the number of cells served by the RBS, as a DL cell requires continuous transmission of cell defining signals, such as reference signals. Furthermore, in UL, the RE forwards a continuous flow of In phase/Quadrature (IQ) samples to the REC for baseband processing, even though no user related data is received in the RE.